The etiologies of dental caries and periodontal diseases are closely associated with dental plaque, the biofilm that develops from microbial colonization of the tooth surface. A limited number of gram-positive species, primarily viridans group streptococci and actinomyces initiate colonization. These bacteria typically occur in vivo as members of a complex microbial community, the formation of which depends on an array of specific adhesive interactions. Adhesins that contribute to these interactions are associated with proline rich protein (PRP)-binding type 1 fimbriae and Gal/GalNAc-binding type 2 fimbriae of Actinomyces naeslundii and with the PRP- and sialic acid binding activities of Streptococcus sanguis or Streptococcus gordonii. Potential salivary receptors for these adhesins have now been assessed by the bacteria overlay technique. Bacteria with PRP-binding adhesins recognized a wide range of parotid and submandibular-sublingual (SM-SL) salivary components. In contrast, bacteria with sialic acid- or Gal/GalNAc-binding adhesins recognized a few SM-SL salivary components, primarily the low molecular weight salivary mucin. Significantly, bacteria that lacked PRP-binding and these lectin-like activities failed to bind any salivary component. Thus, bacterial binding to a surprisingly wide range of surface-associated salivary proteins and glycoproteins depends on a limited number of specific adhesins. The sialic acid-binding adhesin of S. gordonii DL1 was previously associated with the gene hsa, which encodes a 203-kDa serine-rich repeat protein. The predicted protein sequence consists of an N-terminal non repetitive region (NR1) that includes a signal sequence, a relatively short serine-rich region (SR1), a second non repetitive region (NR2), a long serine-rich region (SR2) containing 113 dodecapeptide repeats, and a C-terminal cell wall anchoring domain. The contributions of SR1, NR2 and SR2 to Hsa-mediated adhesion have now been assessed by genetic complementation. Adhesion of an hsa chromosomal deletion mutant to sialic acid-containing receptors was restored by plasmids containing hsa constructs that encoded SR1, NR2 and either the N- or C-terminal portion of SR2, in addition to NR1 and the cell wall anchoring domain. In contrast, hsa constructs that lacked the coding sequences of SR1, NR2 or the entire SR2 region failed to restore adhesion. Surface expression of recombinant Hsa was unaffected by the removal of SR1, NR2 or a portion of SR2, but greatly reduced by the complete removal of SR2. Wheat germ agglutinin, a probe for Hsa-specific glycosylation, reacted with recombinant Hsa that lacked SR1, NR2 or SR2, but not with recombinant Hsa that lacked both SR1 and SR2. Significantly, the aggregation of human platelets by S. gordonii DL1, an interaction implicated in the pathogenesis of infective endocarditis, required the expression of hsa. Moreover, neuraminidase-treatment of the platelets abolished this interaction, further supporting the essential role of Hsa in the bacteria-platelet interaction. Information gained from these studies should contribute to the development of improved approaches for the prevention of plaque-related diseases.